Use SSPP_NONE in mdp5_plane_pipe() if there is now hwpipe allocated for
the drm_plane. Returning '0' means we are returning VIG0 pipe.

Also, use the mdp5_pipe enum to pass around the stage array. Initialize
the stage to SSPP_NONE by default.

We do the above because 1) Cursor plane has to be staged at the topmost
blender of the LM, which can result in empty stages in between 2) In
the future, when we support multiple LMs per CRTC. We could have stages
which don't have any pipe assigned to them.
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>